ORLAND, Calif. -- U.S. Federal safety investigators say the driver of a Fedex tractor-trailer that struck a bus carrying high school students didn't appear to brake before a fiery collision that left 10 dead.

National Transportation Safety Board member Mark Rosekind said Saturday that the truck left no tire marks as it careened across a median and slammed into the bus taking the students to a college tour in Northern California.

Investigators also interviewed a witness who said the truck was already on fire before it crossed the median.

The truck and bus exploded into towering flames after Thursday's crash, making it difficult for investigators to determine whether a fire started in the truck. But Rosekind says investigators plan to look at a blood test and other physical evidence to make that determination.

Related Stories

Photos

Candles and flowers are seen at a make-shift memorial for the victims of a multi-vehicle accident that included a tour bus and a FedEx truck near Orland, Calif. on Friday, April 11, 2014. (The Sacramento Bee / Hector Amezcua)

A California Highway Patrol officer walks past the charred remains of a tour bus at a CalTrans maintenance station in Willows, Calif., Friday, April 11, 2014. (AP / Jeff Chiu)

A California Highway Patrol officer walks past the demolished cab of a FedEx truck at a CalTrans maintenance station in Willows, Calif., Friday, April 11, 2014. (AP / Jeff Chiu)

Students Mary Jeanne Arriaga, left, and Heidi Mendez, react after learning about the death of student Adrian Castro, as they leave El Monte High School in El Monte, Calif., Friday, April 11, 2014. (AP / Damian Dovarganes)